djpeg

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examples

0

This example decompresses the JPEG file foo.jpg, quantizes it to
256 colors, and saves the output in 8-bit BMP format in foo.bmp:

description

djpeg
decompresses the named JPEG file, or the standard input if
no file is named, and produces an image file on the standard
output. PBMPLUS (PPM/PGM), BMP, GIF, Targa, or RLE (Utah
Raster Toolkit) output format can be selected. (RLE is
supported only if the URT library is available.)

options

All switch
names may be abbreviated; for example,
-grayscale may be written -gray or
-gr. Most of the "basic" switches can
be abbreviated to as little as one letter. Upper and lower
case are equivalent (thus -BMP is the same as
-bmp). British spellings are also accepted
(e.g., -greyscale), though for brevity these
are not mentioned below.

The basic
switches are:
-colorsN

Reduce image to at most N
colors. This reduces the number of colors used in the output
image, so that it can be displayed on a colormapped display
or stored in a colormapped file format. For example, if you
have an 8-bit display, you’d need to reduce to 256 or
fewer colors.

-quantizeN

Same as -colors.
-colors is the recommended name,
-quantize is provided only for backwards
compatibility.

Force gray-scale output even if
JPEG file is color. Useful for viewing on monochrome
displays; also, djpeg runs noticeably faster in this
mode.

-scaleM/N

Scale the output image by a
factor M/N. Currently the scale factor must be 1/1, 1/2,
1/4, or 1/8. Scaling is handy if the image is larger than
your screen; also, djpeg runs much faster when
scaling down the output.

-bmp

Select BMP output format (Windows flavor). 8-bit
colormapped format is emitted if -colors or
-grayscale is specified, or if the JPEG file is
gray-scale; otherwise, 24-bit full-color format is
emitted.

-gif

Select GIF output format. Since GIF does not support
more than 256 colors, -colors 256 is assumed
(unless you specify a smaller number of colors).

Select PBMPLUS (PPM/PGM) output format (this is the
default format). PGM is emitted if the JPEG file is
gray-scale or if -grayscale is specified;
otherwise PPM is emitted.

-rle

Select RLE output format. (Requires URT library.)

-targa

Select Targa output format. Gray-scale format is emitted
if the JPEG file is gray-scale or if -grayscale
is specified; otherwise, colormapped format is emitted if
-colors is specified; otherwise, 24-bit
full-color format is emitted.

Switches for
advanced users:
-dct int

Use integer DCT method
(default).

-dct fast

Use fast integer DCT (less
accurate).

-dct float

Use floating-point DCT method.
The float method is very slightly more accurate than the int
method, but is much slower unless your machine has very fast
floating-point hardware. Also note that results of the
floating-point method may vary slightly across machines,
while the integer methods should give the same results
everywhere. The fast integer method is much less accurate
than the other two.

-dither fs

Use Floyd-Steinberg dithering
in color quantization.

-dither
ordered

Use ordered dithering in color
quantization.

-dither none

Do not use dithering in color
quantization. By default, Floyd-Steinberg dithering is
applied when quantizing colors; this is slow but usually
produces the best results. Ordered dither is a compromise
between speed and quality; no dithering is fast but usually
looks awful. Note that these switches have no effect unless
color quantization is being done. Ordered dither is only
available in -onepass mode.

-mapfile

Quantize to the colors used in
the specified image file. This is useful for producing
multiple files with identical color maps, or for forcing a
predefined set of colors to be used. The file must be
a GIF or PPM file. This option overrides
-colors and -onepass.

-nosmooth

Use a faster, lower-quality
upsampling routine.

-onepass

Use one-pass instead of
two-pass color quantization. The one-pass method is faster
and needs less memory, but it produces a lower-quality
image. -onepass is ignored unless you also say
-colorsN. Also, the one-pass method is
always used for gray-scale output (the two-pass method is no
improvement then).

-maxmemoryN

Set limit for amount of memory
to use in processing large images. Value is in thousands of
bytes, or millions of bytes if "M" is attached to
the number. For example, -max 4m selects
4000000 bytes. If more space is needed, temporary files will
be used.

-outfilename

Send output image to the named
file, not to standard output.

-verbose

Enable debug printout. More
-v’s give more output. Also, version
information is printed at startup.

-debug

Same as -verbose.

environment

JPEGMEM

If this environment variable is set, its value is the default
memory limit. The value is specified as described for the
-maxmemory switch. JPEGMEM overrides the default
value specified when the program was compiled, and itself is
overridden by an explicit -maxmemory.

hints

To get a quick preview of an image, use the -grayscale
and/or -scale switches. -grayscale -scale 1/8 is
the fastest case.

Several options are available that trade off image quality to
gain speed. -fast turns on the recommended settings.

-dct fast and/or -nosmooth gain speed at a small
sacrifice in quality. When producing a color-quantized image,
-onepass -dither ordered is fast but much lower quality
than the default behavior. -dither none may give
acceptable results in two-pass mode, but is seldom tolerable in
one-pass mode.

If you are fortunate enough to have very fast floating point
hardware, -dct float may be even faster than -dct
fast. But on most machines -dct float is slower than
-dct int; in this case it is not worth using, because its
theoretical accuracy advantage is too small to be significant in
practice.

bugs

To avoid the
Unisys LZW patent, djpeg produces uncompressed GIF
files. These are larger than they should be, but are
readable by standard GIF decoders.